Microcline, like soda-orthoclase, contains both potash and soda It is, however, triclinic, but only slightly so. As it possesses a notable amount of potash, it may be classified with soda-orthoclase as represent ing the transition' varieties of feldspar, lying between the monoclinic, potash, orthoclase series and the triclinic, soda-lime, plagioclase series.
The more typical plagioclases are repre sented by albite, oligoclase, andesine, labradonte, bytownite and anorthite, which together con stitute the albite-anorthite series. The theory of their constitution was propounded by Tscher mark in 1864. He showed that the series can best be explained as a mixture of two extreme molecules, the one a pure soda-alumina silicate molecule, and the other a lime-alurnima sili cate molecule, found in anorthite. The inter mediate varieties can be considered as varying mixtures of these two molecules.
The feldspars are not only of primary im portance as rock-formers, but the classification of rocks as at present accepted (more particu larly the igneous varieties) depends largely upon the kind of feldspar which they contain.
The feldspars can be grouped in tabular form thus: Feldspathoids.— The minerals of this group are called feldspathoids or feldspar-like minerals, more particularly because of their chemical resemblance to the feldspars. There are but two species which need be considered here: leucite, which has a chemical composi tion expressed by the formula ICA1(Si01),: and nohelite, having the composition KeNa.AleSisOsk Chemically leucite •resembles orthoclase feldspar. In addition to being chem ically like orthoclase, leucite, like its counter part, alters to kaolin. But more interesting still is the fact that on decomposition it has been known to furnish orthoclase or orthoclase and muscovite.
Nephelite, as its chemical composition indi cates, is analogous to the plagioclase feldspars. It is hexagonal in character. The decomposi tion of nephelite, like that of the plagioclases, usually results in the formation of some one of the zeolites, or more rarely it forms kaolin.
The feldsparthoids are lower in silica than their corresponding feldspars, and it is be lieved that, where they are present, true feld spars would have formed, except for this de ficiency in silica. They hold an important place
in the classification of rocks.
Mica.— Two species here are of widespread occurrence; one is muscovite, the white, silvery, potash variety, having a chemical composition corresponding to Chemically it is closely allied to orthoclase, and frequently results as an alteration product of that mineral. The second species, biotite, is dark-colored, owing to comparatively high percentages of iron. It also contains -varying amounts of and might thus be properly classed the following group of minerals. It has the chemical composition (HK)*(MgFe), (AlFe).(SiO4)s.
The Ferromagnesian Minerals.— The min erals of this group take their name from the fact that they contain iron and magnesia as two of their prominent constituents. In addition they may contain lime and alumina in consider able amounts, also some alkali. The more im portant members of the group are included un der the amphiboles and the pyroxenes, two species having numerous varieties. They form two parallel series which are both chemically and crystallographically analogous. Their re lationship can best be shown by the following table: One other important rock-former must be mentioned as belonging to this group — olivine, so. called on account of its usual olive-green color. It has the chemical composition (Mg,Fe)iSiO4, is orthorhombic in crystalliza tion, and readily decomposes to serpentine.
Ores.— Minerals of this group that consti tutes important rock-formers ,usually have iron as their principal constituent, and frequently constitute ores of that substance. Included here are: Thus far the mineral species enumerated are mostly found as the essential constituents of that class of rocks hereafter to be described as igneous.
Accessory Minerals.— Those minerals which are usually of subordinate importance, and which are, therefore, more in the nature of accessory than notable rock-constituents, are included here: Some of the above minerals occasionally come to be of considerable importance, or may even constitute the hulk of the rock-mass. As a rule, however, they are of minor considera tion and frequently even negligible.